feat(browser)!: require a provider factory instead of a string

[sheremet-va]

Description

This PR introduces a new way to define a provider in the browser mode:

import { playwright } from '@vitest/browser/providers/playwright'

export default defineConfig({
  test: {
    browser: {
      // shared provider options between all instances
      provider: playwright({
        launchOptions: {
          slowMo: 50,
          channel: 'chrome-beta',
        },
        actionTimeout: 5_000,
      }),
      instances: [
        { browser: 'chromium' },
        {
          browser: 'firefox',
          // overriding options only for a single instance
          // this will NOT merge options with the parent one
          provider: playwright({
            launchOptions: {
              firefoxUserPrefs: {
                'browser.startup.homepage': 'https://example.com',
              },
            },
          })
        }
      ],
    },
  },
})

You don't have to provide any custom options:

import { playwright } from '@vitest/browser/providers/playwright'

export default defineConfig({
  test: {
    browser: {
      provider: playwright(),
      instances: [
        { browser: 'chromium' },
      ],
    },
  },
})

The main goal of this PR is to make configuring the provider easier. Right now users have to specify the @vitest/browser/providers/playwright module in tsconfig or a <reference> comment.

By importing a function directly, we also auto load the types. This makes the /// <reference comment irrelevant.

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. If the feature is substantial or introduces breaking changes without a discussion, PR might be closed.
• Ideally, include a test that fails without this PR but passes with it.
• Please, don't make changes to pnpm-lock.yaml unless you introduce a new test example.
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Tests

• Run the tests with pnpm test:ci.

Documentation

• If you introduce new functionality, document it. You can run documentation with pnpm run docs command.

Changesets

• Changes in changelog are generated from PR name. Please, make sure that it explains your changes in an understandable manner. Please, prefix changeset messages with feat:, fix:, perf:, docs:, or chore:.

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